UCMP MYSTERY FOSSIL #58 is . . .
the eurypterid Eurypterus lacustris

Eurypterids are arthropods, like insects and spiders, but the spiny appendages that appear to be mouthparts are actually legs. Eurypterid fossils first appear in the Ordovician  the group reached its peak of diversity in the Silurian and Lower Devonian, and then went extinct by the close of the Permian. Some eurypterids reached about six feet in length, making them the largest known arthropods that ever lived.

These specimens are from the Silurian Williamsville Formation of Erie County, New York. Read more about
eurypterids here.